Add multiple decoration, fix interim title change

Add support for having multiple Hooker decorators per function. This is
especially useful for WM hooks, that might want to use the same runner for
multiple events.

Extend hlwm hook to have support for multiple decorations.

Add window_title_changed event to windowtitle changer, effectively making the
windowtitle value update on tab changes in your browser or when you switch
buffers in vim.
Add urgent flag handling to tags() runner

The method for detecting urgency kind of changed with the multiple hook support
since the hooks now only look for the complete event rather than just using
.startswith(). This made the urgent handling stop working. This commit re-adds
it.
Merge pull request #22 from thiderman/multideco

Add support for multiple decorators